The invention relates to a method for operating an exhaust aftertreatment system and an exhaust aftertreatment system.
Both carbon particulates and nitrogen oxides such as NO and NO2, also referred to as NOx, are typical emissions in the exhaust gas of diesel engines. Requirements for reducing such emissions increase, and trigger various approaches in the art to reduce emissions. In the European patent EP 1 054 722 B1 an exhaust aftertreatment system is disclosed which combines a particulate filter collecting soot and nitrogen oxides reducing catalysts in the exhaust tract. For removing soot NO2 is generated by oxidation of NO in an oxidation catalyst. Soot which is collected in a particulate filter is oxidized by NO2. Residual amounts of NO and NO2 in the exhaust gas are reduced to nitrogen gas in a selective-catalytic-reduction catalyst (SCR catalyst) by injecting ammonia into the SCR catalyst. The ratio of NO2 and NO in the exhaust gas is adjusted by using an appropriate oxidation catalyst for a particular SCR catalyst. For instance, Pt/AI2O3 oxidation catalysts with different Pt contents produce different NO2/NO ratios. For a metal/zeolite SCR catalyst all NO should be oxidized to NO2, and for a rare-earth-based SCR catalyst a high NO2/NO ratio is desirable, whereas for transition-metal-based SCR catalysts gas mixtures of NO2 and NO are preferred instead of pure or mainly NO2 or NO gases.
The design of the oxidation catalyst usually has to be a compromise between an optimal passive burning of soot in the particulate filter and an optimal conversion of NO and NO2 in the SCR catalyst. For instance, at certain engine loads only an insufficient amount of NO is oxidized to NO2 resulting in that the particulate filter will be filled with soot and that the SCR catalyst's efficiency is low due to a surplus of NO. At other engine loads the NO2 formation in the oxidation catalyst will be too high resulting in a NO2 surplus into the SCR unit resulting in NO2 and N2O emissions. The exhaust gas composition varies strongly at different engine loads. The concurring processes described above yield only a narrow range of satisfying simultaneous soot oxidation and NOx conversion with respect to engine load and the resulting varying amounts of different kinds of constituents in the exhaust gas.
It is desirable to provide an improved method for operating an exhaust aftertreatment system for a wider range of engine loads and exhaust gas compositions. It is desirable to provide an improved exhaust aftertreatment system which can handle the exhaust gas produced during a wide range of engine loads and exhaust gas compositions.
According to a first aspect of the invention, a method is proposed for operating an exhaust aftertreatment system of an engine, particularly a diesel engine, in which one or more constituents of the exhaust gas are oxidized in an oxidation catalyst and one or more constituents of the exhaust gas are deoxidized by means of a group of possible chemical reactions of different type between the one or more constituents of the exhaust gas and catalytic material arranged in a selective-catalytic-reduction catalyst, wherein the exhaust gas flows from the oxidation catalyst to the selective-catalytic-reduction (SCR) catalyst. The oxidation catalyst can be a separate device or can be part of a diesel particulate filter. According to the invention at least one desired ratio among one or more pairs of the one or more constituents is adjusted by varying a space velocity of the exhaust gas in at least the oxidation catalyst; the space velocity of the exhaust gas is varied by varying one or more operation parameters of the engine; and the ratio is established to a value at least approaching the desired ratio among the one or more pairs of the one or more constituents at the inlet of the selective-catalytic-reduction catalyst.
Favourably, it is possible in many load conditions of the engine to provide an exhaust gas mixture to the SCR catalyst which allows an efficient and fast removal of nitrogen oxides out of the exhaust gas, while NO2 is produced to oxidize soot trapped in a particulate filter in the exhaust gas line.
Generally, the space velocity in a chemical reactor design represents the relation between a volumetric flow of a feed and a reactor volume. The space velocity indicates how many reactor volumes of feed can be treated in a unit time. It is possible to change the exhaust gas flow by changing the air intake flow of the air which the engine takes in.
If the desired ratio among the constituents can be established in the exhaust gas, it can be tried to establish the ratio among the constituents so that at a given reaction temperature in the SCR catalyst one specific chemical reaction is selected out of a group of possible chemical reactions which can take place among the constituents of the exhaust and the catalyst material in the SCR catalyst, wherein the selected specific chemical reaction has a higher probability to be performed than each single one of the other chemical reactions.
Preferably, the space velocity can be varied by of the exhaust gas by varying an air intake flow into the engine. There exist a number of possible measures to vary the air intake flow, which can be applied individually or in appropriate combinations of at least two such measures:
It is possible to vary the air intake flow by varying an intake pressure by at least one of adjusting a turbine geometry of a variable turbine in the air intake flow, adjusting a throttle in the air intake flow. If a higher pressure is applied, more exhaust gas is produced.
Alternatively or additionally, the air intake flow can be varied by varying one or more intake valves of the engine. Closing an intake valve, when its piston is near the top, results in less air compared to closing the valve when its piston is near the bottom. Alternatively or additionally the air intake flow can be varied by varying an amount of exhaust gas in an exhaust gas recirculation. An increase of the amount of recirculated exhaust gas results in less fresh air in the exhaust.
Near maximum load conditions of the engine there is only a minimum room for varying the intake air flow since a maximum air flow is needed to give a good combustion of the fuel which is fed into the engine. On the other hand the temperature is typically high enough so that NO2 and NO will reach equilibrium in the oxidation catalyst regardless of the space velocity of the exhaust gas. At conditions with lower load of the engine there is more room for changing the intake air flow with a satisfying good combustion. A variation of the air intake flow can change the NOx and soot content and the temperature in the exhaust gas as well as the fuel consumption of the engine. NOx and soot variations can be easily adjusted to appropriate levels with regard if the SCR catalyst and a particulate filter by changing the timing of the injection of fuel into the engine. Further it is preferred to keep fuel consumption and total emissions on a satisfying level when the air intake flow is altered by adjusting appropriate engine parameters such as timing of fuel injection and/or pressure of the fuel injector.
Favourably, the desired ratio among the one or more constituents is a ratio of NO2/NO close to 1 and preferably not exceeding 1, particularly a ratio of NO2/NO=0.8±0.2, preferably NO2/NO=0.9±0.1, most preferably NO2/NO=0.951±0.05. By choosing a ratio close to 1 it is possible to trigger a fast and highly efficient chemical reaction which reduces NO as well as NO2 and NH3 to N2 gas and water in the presence of the SCR catalyst. This reaction is favourable for a wide range of exhaust gas temperatures from below 200° C. and above. Other chemical reactions are possible depending on the amount of NO2 and NO, i.e. ratio of NO2/NO, present in the SCR catalyst. These reactions, however, are typically slower and prone to competitive reactions producing N2O and the like. Favourably, the efficiency of the selective catalytic reduction of the constituents of the exhaust gas can be optimized while at the same time good operating conditions can be provided for a particulate filter arranged upstream of the SCR catalyst. Preferably the particulate filter is arranged between the oxidation catalyst and the SCR catalyst. Alternatively or additionally, the oxidation catalyst can be at least partially be integrated in the particulate filter as an oxidation catalyst coating. The operating region where the exhaust aftertreatment system operates well can be enlarged compared to the prior art system which operates well only close to a few operating points of the engine. The method allows for an efficient exhaust aftertreatment with respect to cost, packaging and durability.
Additionally, the space velocity can be varied by controlling the flow of the exhaust gas through the oxidation catalyst which can be done by using an external bypass and/or an internal bypass inside the oxidation catalyst which allows to varying the flow distribution to the catalyst. The flow distribution may be varied by e.g. covering parts of the catalyst thus blocking catalyst against the exhaust gas, using flow guides for directing the exhaust gas and/or by opening valves that cover inlet and/or outlet ports in the oxidation catalyst. This may also be combined with a non-uniform distribution of the catalytically active material over the catalyst for further increasing the effect. Generally, an external bypass can be provided combined with the possibility to vary the space velocity of the exhaust gas flow.
Preferably, the ratio among the one or more constituents is established depending on the amount of soot which is contained in a particulate filter arranged upstream of the selective-catalytic-reduction catalyst.
According to a preferred development, the ratio among the constituents can additionally or alternatively be established depending on the amount of soot which is contained in a particulate filter arranged between the oxidation catalyst and the SCR catalyst. NO2 which is generated in the oxidation catalyst oxidizes soot trapped in the particulate filter. The amount of NO2 needed varies with the amount of soot in the particulate filter. Advantageously, the ratio among the members of the constituents can be established depending on the amount of NO2 which is generated in the particulate filter. The particulate filter can comprise an oxidation catalyst and thus produce NO2 which adds to the NO2 generated in the oxidation catalyst.
According to a preferred further development, additionally or alternatively the ratio among the constituents is established depending on the amount of NO2 which is generated in the oxidation catalyst and/or over the catalytic coating of the particulate filter. The oxidation catalyst can generate NO2 for both the passive oxidation of soot in the particulate filter as well as for the selective catalytic reduction in the SCR catalyst. The NO2 generated in the particulate filter is reacting back to NO on the soot so that the amount of NO2 and NO formed in the particulate filter is strongly dependent on the condition of the particulate filter, e.g. the amount of soot and on the reaction temperature, i.e. the exhaust temperature, wherein the selected specific chemical reaction has a higher probability to be performed than each single one of the other chemical reactions.
The ratio among the constituents can additionally or alternatively be established depending on the amount of sulphur which is adsorbed in the oxidation catalyst and/or on a oxidation catalytic coating of the particulate filter. The oxidation catalyst absorbs sulphur at lower exhaust gas temperatures and releases the sulphur at temperatures above 350° C. If operating conditions of the engine let the oxidation catalyst adsorb a lot of sulphur contained in the exhaust gas, the NO2 formation in the oxidation catalyst will be poisoned.
Favourably additionally or alternatively, the ratio among the constituents can be established depending on the amount of ammonia which is provided in the SCR catalyst. On an SCR catalyst ammonia is reacting with NOx to form nitrogen. On vehicles urea is injected into the exhaust gas and by the exhaust temperature urea is thermolyzed and/or hydrolyzed to ammonia in the exhaust gas and on the catalyst.
According to a further aspect of the invention, an exhaust aftertreatment system comprising at least an oxidation catalyst and a selective-catalytic-reduction catalyst arranged in an exhaust line of an engine, which is operated with a method as described previously, wherein at least one desired ratio among one or more pairs of the one or more constituents is adjustable by varying a space velocity of the exhaust gas in at least the oxidation catalyst; the space velocity of the exhaust gas is variable by varying one or more operation parameters of the engine; and the ratio can be established to a value at least approaching the desired ratio among the one or more pairs of the one or more constituents at the inlet of the selective-catalytic-reduction catalyst.
The space velocity of the exhaust gas in the oxidation catalyst and/or the portion of exhaust gas which can be fed into the bypass line and the portion of exhaust gas which can be fed into the oxidation catalyst can be controlled depending on operating parameters of the engine and/or on operating parameters of one or more catalysts arranged in the exhaust aftertreatment system. Consequently, a NOx or NO2 sensor can be replaced by a virtual sensor which uses a model of the engine and the exhaust aftertreatment system to calculate the relevant parameters, particularly the NO2 and NO content in the exhaust gas at the inlet of the SCR catalyst. Preferably parameters are available such as exhaust gas flow, temperatures in the oxidation catalyst and particulate filter, NO and NO2 flow from the engine, soot flow from the engine and/or soot load in the particulate filter. Some of the parameters can be measured and other parameters can be calculated from other sensors and engine parameters.
Preferably a particulate filter can be arranged upstream of the selective-catalytic-reduction catalyst. Particularly, the particulate filter can be arranged downstream of the oxidation catalyst and/or can comprise an oxidation catalyst coating.
According to a further aspect of the invention, a computer program storable on a computer readable medium is proposed, comprising a program code for use in a method comprising at least the steps of adjusting at least one desired ratio among one or more pairs of one or more constituents by varying a space velocity of exhaust gas in at least an oxidation catalyst stage; varying the space velocity of the exhaust gas by varying one or more operation parameters of an engine; and establishing the ratio to a value at least approaching the desired ratio among the one or more pairs of the one or more constituents at an inlet of a selective-catalytic-reduction catalyst.
In the drawings, equal or similar elements are referred to by equal reference numerals. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. Moreover, the drawings are intended to depict only typical embodiments of the invention and therefore should not be considered as limiting the scope of the invention.